A radiation detector using a solid state device, such as an image sensor, has recently been under development as a radiation detector which detects radiation, such as X-rays, instead of a conventional radiation detection system using an intensifying screen and an X-ray film.
Radiation detectors (radiation image capturers) using a thin film transistor (TFT) panel, in particular, are being actively developed because the radiation detectors have the advantage of being lensless and the advantage of being suitable for large-screen image capturing, as compared to an image sensor, such as a charge coupled device (CCD) or a complementary metal-oxide semiconductor device (CMOS).
A radiation detector using a TFT panel includes an image capturing circuit having a large number of pixels two-dimensionally arranged. Each pixel converts a physical quantity corresponding to the dose of X-rays (the dose of radiation) into a signal and measures the signal obtained through the conversion, thereby performing image capturing.
Methods for reading out signals obtained through conversion at pixels in the radiation detector using the TFT panel are broadly classified into two types.
One method is reading out the amount of charge (the amount of carriers) stored at each pixel without any change and is called a passive pixel sensor (PPS) method.
The other method is generating a potential or a current corresponding to the amount of charge stored at each pixel and reading out the generated potential or current and is called an active pixel sensor (APS) method.
The potential of each pixel is reset with each readout in a panel based on the PPS method of the two methods. The panel based on the PPS method is thus relatively easy to use and has been put into practical use ahead of the APS method. Note that a readout circuit for the PPS method for reading out the amount of charge stored at each pixel has also been put into practical use. The readout circuit for the PPS method is generally created by a CMOS process or the like separately from the TFT panel and is connected to the TFT panel via a flexible board.
FIG. 7 is an explanatory diagram showing an example of the configurations of one pixel 100 and a readout circuit 120 connected to the pixel 100 in a conventional radiation detector based on the PPS method.
The pixel 100 includes a photodiode 101, a charge storage part (charge storage node) 102, and a readout switch 103. The photodiode 101 photoelectrically converts light (radiation) in a predetermined frequency band which is incident from the outside and stores charge generated through the photoelectric conversion in the charge storage part 102. With this configuration, charge corresponding to the amount of light incident on the photodiode 101 is stored in the charge storage part 102.
One end side of the readout switch 103 is connected to the charge storage part 102, and the other end side is connected to a signal output line 110. The readout switch 103 switches between a state of continuity between the charge storage part 102 and the signal output line 110 and a state of discontinuity in accordance with an instruction from control means (not shown).
The readout circuit 120 includes an amplifier reset switch 121, a feedback capacitor 122, and a readout amplifier 123. The signal output line 110, one end side of the feedback capacitor 122, and one end side of the amplifier reset switch 121 are connected to an input terminal of the readout amplifier 123. A readout amplifier output line 130, the other end side of the feedback capacitor 122, and the other end side of the amplifier reset switch 121 are connected to an output terminal of the readout amplifier 123.
If the readout switch 103 is switched to the state of continuity, charge corresponding to the amount (Qsig) of charge stored in the charge storage part 102 is stored in the feedback capacitor 122 (CF) connected in parallel with the readout amplifier 123. As a result, an output potential Vout from the readout amplifier 123 to the readout amplifier output line 130 is an output potential corresponding to the amount of charge stored in the photodiode 101, as given by Expression (1) below.Vout=Qsig/CF  (1)
At this time, the potential of the signal output line 110 is set to a predetermined potential through feedback from the readout amplifier 123. After the readout, the readout switch 103 is opened (turned off), continuity between the charge storage part 102 and the signal output line 110 is broken, and signals obtained through photoelectric conversion are stored in the charge storage part 102 again.
To implement a further reduction in the dose of a detectable range or a further increase in the resolution in the detectable range in a radiation detector, an SN ratio needs to be increased. For an increase in the SN ratio, the APS method is considered promising, as described in, for example, PTL 1.
In the PPS method, an output from each pixel is a charge amount. In the APS method, an output is generally a current amount. A charge integration circuit is generally used as a readout circuit which reads out an output from each pixel.
Since a signal is charge itself stored at a pixel in the PPS method, a readout circuit is required to have the ability to accurately read out charge.
In contrast, in the case of the APS method, an output signal from a pixel is a current. Lengthening of an integral time leads to obtainment of a larger signal than in a readout circuit based on the PPS method if the efficiency of conversion from the same amount of light (radiation) into charge is the same.
However, there is a problem in that, when a radiation detector based on the APS method is operated, noise occurs in a finally obtained image if there is variation in initial state among pixels. For this reason, in the radiation detector based on the APS method, pixels are need to be initialized (reset) at the beginning of use or periodically.
Note that, for example, the method disclosed in PTL 2 can be used as a method for determining an operating point at the time of the initialization operation. The method in PTL 2 is giving feedback such that an output from a given pixel is equal to a reference voltage when the pixel is selected. In the method in PTL 2, a current flowing in a pixel is determined by a load transistor.